Brake Disc and Method for Manufacturing a Brake Disc

ABSTRACT

A brake disc includes at least one friction surface consisting of a base body. The base body can be made from gray cast iron. At least one coating is applied on at least parts of the friction surface. The coating contains at least tungsten chromium carbide 2C and nickel-chromium NiCr. The coating may further contain tungsten carbide WC. The production of the brake disc according to the disclosure is also described herein. The coating has excellent oxidation resistance and good wear resistance even at high temperatures of up to 800° C.

The present invention relates to a brake disk and a process forproducing a brake disk having the features of the independentlyformulated claims.

Brake disks have the task of transmitting the braking momentum to theaxle on slowing of the vehicle and also temporarily storing the kineticenergy arising during braking for a short time in the form of heat andsubsequently passing it on again by conduction, radiation andconvection.

Conventional brake disks are frequently made of the heat-resistant graycast iron. Wear-resistant brake disks are produced by either using veryexpensive solid ceramic brake disks or spraying a wear protection layeronto a gray cast iron (GCI) disk by means of thermal spraying processes,for example HVOF processes. WC-based powder is frequently used for thispurpose and leads to high wear resistance. Such a brake disk isdisclosed in DE 10 2011 087 136 A1.

As mentioned above, heat is generated on braking and is stored in thebrake disk. The temperature increases in extreme cases, i.e. brakingfrom full speed, up to 800° C.

For this reason, the wear protection layers of the cast iron brake diskshave to have sufficient wear resistance at high temperatures up to 800°C.

DISCLOSURE OF THE INVENTION

The invention proceeds from a brake disk comprising at least onefriction surface consisting of a main element. The main element can bemade of gray cast iron. At least one coating is applied to at leastparts of the friction surface.

The key idea of the invention is that the coating contains at leasttungsten-chromium carbide (W, Cr)₂C and nickel-chromium NiCr.

In a particularly advantageous embodiment of the invention, the coatingadditionally contains tungsten carbide WC.

Tungsten-chromium carbide (W, Cr)₂C is tungsten dicarbide W₂C into whichchromium has been incorporated in various amounts. This mixed carbide istherefore referred to as (W, Cr)₂C. This phase has excellent oxidationresistance and good wear resistance even at high temperatures up to 800°C. In addition, the high-temperature strength and high-temperatureoxidation resistance of nickel are significantly increased by alloyingwith chromium.

In the coating, tungsten carbide WC is advantageously essentiallyuniformly dispersed in a matrix phase composed of tungsten-chromiumcarbide (W, Cr)₂C and nickel-chromium NiCr.

Above a braking temperature of about 600° C., a very thin andimpermeable oxide layer which can contain, in particular, at leastnickel-tungsten oxide NiWO₄ and/or chromium-tungsten oxide CrWO₄ and/orchromium oxide Cr₂O₃ and/or tungsten oxide WO₃ will advantageously beformed on the layer surface. The formation of the oxide layerconsiderably slows the further oxidation process of the wear protectionlayer, which very effectively avoids oxidation and a loss of function ofthe coating at high temperatures.

The coating according to the invention can be applied directly to themain element. Here, the coating is, in particular, applied to a workedor modified surface of the main element.

However, the coating according to the invention can also have beenapplied by means of at least one intermediate layer to the main element,with the intermediate layer being able to be a metallic intermediatelayer which contains, for example, nickel Ni. However, the intermediatelayer can also be realized by means of a treatment of the surface of themain element, for example by means of a remelting process.

The invention also provides the production of a brake disk comprising atleast one friction surface consisting of a main element, in particular amain element made of gray cast iron, and at least one coating applied toat least parts of the friction surface.

The key idea of the invention is the step of coating at least parts ofthe friction surface with at least tungsten-chromium carbide ((W, Cr)₂C)and nickel-chromium (NiCr).

Further advantageous embodiments of the invention are indicated in thedependent claims and the working example.

DRAWINGS

Working examples of the invention will be illustrated below with the aidof drawings. The drawings show:

FIG. 1 shows a perforated and coated brake disk as per the prior art.

FIG. 2 shows a polished cross section of a brake disk which has beencoated according to the invention after a severe braking event.

FIG. 3 shows the production process of the invention.

DESCRIPTION OF WORKING EXAMPLES

The brake disk 1 depicted in FIG. 1 comprises a circular disk-shapedbrake ring 2 and a bowl-shaped hub 3 which is made in one piece with andis coaxial with the brake ring 2. The friction ring 2 forms the actualbrake disk. The brake disk 1 is internally ventilated, and its brakering 2 is double-walled. Outer end faces of the brake ring 2 formcircular disk-shaped friction surfaces 4 of the brake disk 1. Thefriction surfaces 4 are the surfaces of the brake disk 1 against whichfriction brake linings (not shown) are pressed during braking in orderto brake the brake disk 1 by friction. The brake disk 1 is perforated,i.e. it has holes 5 which run through the brake ring 2. The brake disk 1consists of gray cast iron or a steel alloy.

The friction surfaces 4 of the brake disk 1 are provided with a surfacecoating which increases wear resistance and corrosion resistance. Thesurface coating can be a thermal powder coating which is applied, forexample, by flame spraying or electric arc spraying. The surface coatingof the friction surfaces 4 can comprise carbides, in particular metalcarbides, for example chromium or tungsten carbides, which are embeddedin a matrix, in particular a metallic matrix composed of, for example,nickel or cobalt. The surface coating of the friction surfaces 4 whichincreases the wear resistance and corrosion resistance can be made up ofone or more layers.

The key idea of the invention in the embodiment depicted is, asindicated above, the use of a wear protection layer consisting oftungsten carbide WC, tungsten-chromium carbide (W, Cr)₂C andnickel-chromium NiCr on cast iron brake disks. (W, Cr)₂C is the tungstendicarbide W₂C which is known per se, into which various amounts ofchromium have been incorporated. This mixed carbide is thereforereferred to as (W, Cr)₂C. This phase has excellent oxidation resistanceand good wear resistance even at high temperatures up to 800° C. Inaddition, the high-temperature strength and high-temperature oxidationresistance of nickel are increased significantly by alloying withchromium. In the coating, WC is uniformly distributed in the matrixphase composed of (W, Cr)₂C and NiCr. Above a brake temperature of about600° C., a very thin impermeable oxide layer which can comprise NiWO₄,CrWO₄, Cr₂O₃ or WO₃ will be formed on the layer surface. The furtheroxidation process of the wear protection layer is considerably slowed bythe formation of this oxide layer, which significantly avoids oxidationand loss of function of the coating at high temperatures.

FIG. 2 shows the polished cross section of a cast iron brake disk coatedwith WC—(W, Cr)₂C—NiCr after a brake fading test above 800° C.

The reference numeral 201 denotes the cast iron main element (etched,with phase transformation), the reference numeral 202 denotes anintermediate nickel layer and 203 denotes the WC—(W, Cr)₂C—NiCr layer.

The depicted oxide layer on the surface of the wear protection layer 203is so thin that it cannot be observed at all in the polished section.After the fading test, the surface of the wear protection layer 203 isstill smooth and the layer thickness is virtually the same as in the newstate, which means that the layer has suffered virtually no wear afterthe test above 800° C.

One embodiment of the cast iron brake disks coated with WC—(W,Cr)₂C—NiCr is direct application of the wear protection layer to thecast iron main element by means of high-velocity flame spraying (HVOF).To ensure good adhesion of the layer to the cast iron main element, thesurface of the cast iron main element can be roughened, e.g. by sandblasting, before HVOF coating.

A further embodiment is to apply a metallic intermediate layer asbonding agent to the cast iron main element before HVOF application ofthe wear protection layer. To this end, preference is given to usingnickel or nickel-based alloys as the metallic intermediate layer.

The preferred compositions of the WC—(W, Cr)₂C—NiCr wear protectionlayer are:

-   -   30-50 percent by weight of WC,    -   20-40 percent by weight of (W, Cr)₂C,    -   10-35 percent by weight of NiCr,    -   and impurities.

The process for producing the brake disk of the invention will now beillustrated with the aid of the figure.

In the (optional) step 801, the main element which in this embodiment isconfigured as gray cast iron is provided. The friction surface of themain element is subsequently treated as indicated above in step 802.

In the (optional) step 803, the friction surface of the main element iscoated with the abovementioned intermediate layer.

In step 804, the coating according to the invention is then applied.

1. A brake disk comprising: at least one friction surface comprising amain element; and at least one coating applied to at least parts of thefriction surface, the coating comprising at least tungsten-chromiumcarbide and nickel-chromium.
 2. The brake disk as claimed in claim 1,wherein the coating further comprises tungsten carbide.
 3. The brakedisk as claimed in claim 2, wherein, in the coating, the tungstencarbide is essentially uniformly dispersed in a matrix phase composed oftungsten-chromium carbide and nickel-chromium.
 4. The brake disk asclaimed in claim 2, wherein the coating further comprises a surfacelayer containing oxides.
 5. The brake disk as claimed in claim 4,wherein the surface layer contains at least one of nickel-tungstenoxide, chromium-tungsten oxide, chromium oxide, and tungsten oxide. 6.The brake disk as claimed in claim 4, wherein the surface layer on thecoating is present only after braking events of particular severity. 7.The brake disk as claimed in claim 1, wherein the coating is applieddirectly to the main element.
 8. The brake disk as claimed in claim 1,wherein the coating is applied to the main element via at least oneintermediate layer.
 9. The brake disk as claimed in claim 8, wherein theintermediate layer contains nickel and/or is formed by a surfacetreatment of the main element.
 10. A process for producing a brake diskcomprising at least one friction surface that includes a main elementand at least one coating applied to at least parts of the frictionsurface, the method comprising: coating at least part of the frictionsurface with at least tungsten-chromium carbide and nickel-chromium toform the at least one coating.
 11. The process as claimed in claim 10,wherein the coating further comprises tungsten carbide.
 12. The processas claimed in claim 11, wherein, in the coating, the tungsten carbide isessentially uniformly dispersed in a matrix phase composed oftungsten-chromium carbide and nickel-chromium.
 13. The brake disk asclaimed in claim 1, wherein the main element is formed of gray castiron.
 14. The brake disk as claimed in claim 7, wherein the coating isapplied to a modified surface of the main element.
 15. The process asclaimed in claim 10, wherein the main element is formed of gray castiron.